To analyze a general sphero-cylindrical ophthalmic lens for power, axis and prism, the prior art is replete with lensometers of various constructions and methods of operation. Generally speaking, a transparent reticular target is interposed between a light source and a collimating lens. The rays pass from the light source through the target and collimating lens and thence through the ophthalmic lens being analyzed. The rays then pass through a telescope which is provided with a prism reticle. The device is usually provided with a pair of manual wheels which translate and rotate the recticular target.
To employ the conventional lensometer the operator looks through the telescope and rotates both wheels slowly and simultaneously in either direction to bring into focus the principle axis line of the target reticle. When the line is judged by the operator to be in focus, the reading on the power wheel indicates spherical power (proportional to translation of the target), and the axis wheel reading indicates the axis of the cylinder. The operator then rotates only the power wheel to bring into focus the orthogonal axis line of the target reticle. This second reading indicates the power in the other principle meridian of the ophthalmic lens. The difference between the first and second power readings is the cylinder of the lens. Thus the ophthalmic lens is analyzed for power, axis and cylindrical correction.
In a professional practice the old and new glasses of each patient are examined for power, axis and prism for both lenses. It is estimated that an average of 20 pairs of glasses per day may be measured in a practitioner's office. In an optical shop where lenses are produced, the lenses are inspected for power, prism and axis during fabrication and again when the lenses are finished. In such a situation it is common to examine an average of 100 pair of glasses per day.
It may be appreciated that the concentration, effort and judgment required to operate the conventional lensometer may result in fatigue to the operator after repetitive, consecutive procedures. This fatigue can affect the judgment and care of the operator and result in substantial error. If several operators use the same lensometer, the telescope and reticle should be recalibrated for each individual to maintain accuracy. This is seldom done in practice, and results in errors in lens measurement.
To reduce the fatigue of monocular viewing in a telescope, projection lensometers have been developed recently. These lensometers permit binocular viewing of the target reticle on a screen. However, all the disadvantages of focusing for each lens are still present. The point of best focus still remains a judgment subject to the condition of visual acuity, concentration, and tedium of the operator.
There are other automatic lensometers; however, all others known are forced to mimic the conventional lensometer by simultaneously seeking both best focus and axis conditions. This requires a complex servo system to form the judgement that the eyes does in a conventional lensometer.
This new device allows axis and power to be automatically and objectively found independently of each other. Both axis and power require only one simple translation or rotation of the target and detector.